1. Field of the Invention
The present invention relates to a group III-V compound semiconductor light emitting device including nitrogen as a major component, and a method for producing the same.
2. Description of the Related Art
Recently, a high-luminance blue light emitting diode made of a GaN compound semiconductor has been put into practice, and a blue laser diode has been vigorously developed. Group III-V nitride compound semiconductors attract much attention as a material for such light emitting devices.
Conventionally, a nitride semiconductor is grown using hydride vapor phase epitaxy (hereinafter referred to as HVPE), metal organic chemical vapor deposition (hereinafter referred to as MOCVD), molecular beam epitaxy (hereinafter referred to as MBE), or the like.
In the case of group III-V nitride compound semiconductors, it is considerably difficult to produce bulk crystal. It is therefore considerably difficult to obtain a group III-V nitride compound semiconductor substrate on which a group III-V nitride compound semiconductor device is grown. For this reason, a sapphire substrate is typically employed. However, a great level of lattice mismatch occurs between GaN (a group III-V nitride compound semiconductor) and a sapphire substrate, e.g., a defect of 109 to 1010/cm2 is present in a GaN film after growth. Such a defect affects the light output or life of a device. To avoid this, GaN is selectively grown to produce a pseudo GaN substrate which is a thick film GaN having a reduced defect. By using such a substrate, a laser device capable of room-temperature continuous-wave operation can be realized.
In addition to defeats residual impurities have an adverse effect on a compound semiconductor light emitting device. The characteristics and the life span of an arsenic-based or phosphorus-based compound semiconductor device are greatly affected by oxygen or carbon atoms contained in the device. Therefore, various attempts have been made to remove such residual impurities.
A residual impurity that causes a problem with a group III-V nitride compound semiconductor light emitting device, is hydrogen. When a nitride compound semiconductor device is grown by the above-described methods, organic metals and ammonia are used as materials of the device. Further, hydrogen or hydride (e.g., hydrogen chloride) is used as a carrier gas.
Therefore, residual hydrogen atoms may be present in a growing film of a nitride compound semiconductor. Particularly, when a p-type layer essential for a nitride compound semiconductor light emitting device is grown, hydrogen atoms are likely to remain in the p-type layer. This is because a hydrogen atom is likely to bind to an Mg atom, a Zn atom, or the like which is a dopant for the p-type layer. For example, Appl. Phys. Lett., Vol. 72 (1998), p. 1748, describes that the residual hydrogen concentration of a growth film of a nitrogen compound semiconductor deposited by MOCVD is 2 to 3×1019 atoms/cm3, where the Mg concentration is 2 to 3×1019 atoms/cm3, and the residual hydrogen concentration increases with an increase in the Mg concentration.
When a hydrogen atom and a p-type dopant such as Mg or Zn bind together, the activity of the dopant is hindered, thereby creating a highly resistant p-type layer.
Japanese Patent No. 2540791 discloses a known technology for preventing a p-type layer from being caused to be highly resistant due to hydrogen atoms. In the technology of Japanese Patent No. 2540791, after growing a group III-V nitride compound semiconductor doped with p-type impurities, annealing is conducted at a temperature of 400° C. or more in an atmosphere not containing hydrogen. The annealing allows hydrogen atoms to be removed from the group III-V nitride compound semiconductor doped with the p-type impurities, thereby obtaining a p-type group III-V nitride compound semiconductor having a low level of resistance.
Hydrogen atoms cannot be sufficiently removed from a p-type layer only by annealing in an atmosphere not containing hydrogen as disclosed in Japanese Patent No. 2540791. Therefore, there is a problem with the technology disclosed in Japanese Patent No. 2540791 in that residual hydrogen atoms in the p-type layer hinder activation of the p-type impurities, and also cause a reduction in the life span of the device. This is because the residual hydrogen atoms are gradually diffused due to the passage of electric current and therefore an active layer is deteriorated. Japanese Patent No. 2540791 does not disclose the atmosphere which is used in the growth of the p-type layer.
Further, when the active layer contains In atoms, Mg and In atoms, as well as hydrogen atoms, are diffused. Especially when the active layer has a thin film quantum well structure, the diffusion of both Mg and In causes considerable deterioration in the active layer.
Furthermore, when the residual hydrogen concentration of an n-type layer is high, the resistance of the n-type layer is also great, thereby deteriorating device characteristics.